1. Field of the Invention
This invention relates generally to a system and method for managing power flow in a fuel cell vehicle that includes providing a power match between actual fuel cell stack power and a fuel cell stack power limit and, more particularly, to a system and method for managing power flow from a fuel cell stack to a controlled electric drive system, sometimes referred to herein as a power inverter module (TPIM), by estimating the unknown offset-power, which is the sum of power of any other load, such as ancillary loads, the distributed efficiency losses and some unmanaged electric loads, for example, heaters, fans, etc., and that employs a single controller for both a power request signal-path and a power limit signal-path.
2. Discussion of the Related Art
Electric vehicles, such as battery electric vehicles (BEV), extended range electric vehicles (EREV), and electric hybrid vehicles that combine a battery and a main power source, such as an internal combustion engine, fuel cell system, etc., exist in the art. Most fuel cell vehicles are hybrid vehicles that employ a rechargeable supplemental high voltage power source in addition to the fuel cell stack, such as a DC battery or an ultracapacitor. The power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. More particularly, the fuel cell stack provides power to a traction motor and other vehicle systems through a DC voltage bus line for vehicle operation. The battery provides the supplemental power to the voltage bus line during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. The fuel cell stack is used to recharge the battery at those times when the fuel cell stack is able to meet the system power demand. The generator power available from the traction motor can provide regenerative braking that can also be used to recharge the battery through the DC bus line.
A typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode side input gas including oxygen, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. The fuel cell stack includes a series of bipolar plates positioned between the several membrane electrode assemblies (MEAs) in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow fields are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow fields are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The propulsion power for operating a vehicle typically is very dynamic. When using a fuel cell stack to supply the vehicle propulsion power, the power provided by the stack is required to follow the dynamic power request from the vehicle driver as provided by the vehicle acceleration pedal. The hydrogen fuel and air (media) are provided to the stack at a certain pressure and flow rate so that when the fuel cell system receives a power request signal, the system controls various devices and components that supply the media to the stack, monitors the stack operating conditions, and provides a power signal that identifies the power provided by the stack. However, there is a limit to the dynamic response of the fuel cell system in that the system cannot follow relatively fast power request transients. In the case of such a transient power request for a vehicle motoring mode, the available power will be provided by the fuel cell stack and the power drawn by the vehicle loads will be required to follow the reduced transient dynamic of the stack power. The physical power flow in the system is actuated by the system loads, usually the vehicle propulsion motor, not by the fuel cell system. However, the set point for the power is either driven by the loads (normal mode), or by the fuel cell system (limited mode).
To prevent damage to the fuel cell stack as a result of a voltage overload, the power drawn by vehicle loads must not exceed an upper power limit provided by the fuel cell stack control. Also, the power drawn from the stack should not fall below the upper power limit for a significant period of time because under-loading the stack could lead to long-term stack degradation. Particularly, if the system devices provide more air and hydrogen than is required for the actual power demand, inappropriate operating conditions of the stack could result, which could lead to long-term degradation as a result of the stack drying out.
As mentioned, the actual stack power during the motoring mode should be maintained as close as possible to the upper power limit. Short-term power deviations are generally acceptable if they are below a certain peak power limit, but it is generally not acceptable if there is a continuous deviation of stack power beyond the upper power limit or that the stack power falls below the lower power limit. If there is a continuous deviation of the stack power from the upper power limit, the system controller must take remedial action. There are two ways to accomplish this control, namely, employ a request power mode where the power consumption from the stack is maintained, and the stack power capability is adjusted by controlling the request signal, or employ a power limit mode where the stack power limit is given by stack operation conditions, and the actual power drawn from the stack is adjusted by controlling the TPIM. If the stack is able to provide the requested power, then the power request mode will be used and the stack power will follow the power request. This means that the power request from the load is controlled in such a manner that the stack power limit matches the actual stack power. If the stack is not able to provide the requested power, then the power limit mode is used and the actual stack power will be maintained at the power limit. This means that the power to the loads is controlled in such a manner that the actual stack power matches the stack power limit all the time. During normal operating conditions, the power request mode is used where the driver sets the stack power by actuating the acceleration pedal, and the power limit is controlled so that it matches the actual power.
The system could also be operated in a regenerative braking (regen) mode where energy from regenerative braking of the vehicle is used to recharge the battery and to supply power to the loads, which saves energy provided by the fuel cell stack and increases the overall system efficiency. In this case, a lower power limit is needed to make sure the stack power does not become negative, where the lower power limit is used for the system control in the same manner as the upper power limit control. The discussion above for the power request mode and the power limit mode described above is only for the motoring mode, and does not consider the regen mode. In the regen mode, the stack power is matched to the lower power limit.
In current fuel cell systems, a separate proportional-integral (PI) controller is used for the power request mode and the power limit mode to match the stack power limit and the actual stack power for the power request and power limit mode. Further, it is necessary to consider the unknown power consumption of the uncontrolled vehicle loads. These loads are not visible to the control system and they are not included in the load control signals, but there power consumption is part of the overall actual stack power. State of the art systems typically measure or estimate how much power each of the auxiliary uncontrolled loads is drawing, and adds those power levels together to get the total power being used. However, it has been difficult to determine how much power is required by the various uncontrolled loads to provide the desired match between the power provided and the power consumed.